1. Field of the Invention
This invention relates to article positioning systems and, more particularly, apparatus for establishing a zero or reference position for such systems.
2. Prior Art
Article positioning systems are known in which an article is supported by an X-Y table and is movable to predetermined positions under the control of some form of a controller or data processing system that provides the necessary commands and signals for controlling the movement of the X-Y table. The systems find use in many diverse applications, the most common being in connection with numerically controlled machine tools or with test apparatus for testing the article being handled. In most of these systems, some form of a zero or reference point is established. Movement of the table relative to the reference point is accomplished through the use of a servo mechanism that provides signals indicating how far the current position of the table is from either the reference point or the point to which it is desired to move the table. An example of the servo controlled positioner is disclosed in U.S. Pat. No. 3,209,222 -- Holy. Another type of positioner that is known in the art is one which uses some form of a scale or graduation that is placed along the path of movement of the structure and is associated with some form of a detection system for reading the scales or graduations so that the position of the article from a reference position is known. An example of a system of this nature is disclosed in U.S. Pat. No. 3,372,321 -- Seiuemon Inaba et al.
Also known under prior art are various forms of electro-optical detectors or sensing devices that are useful for detecting various conditions necessary for determining the relative conditions of two different objects. The above last mentioned patent shows one system in which electro-optical devices are used to read the graduations on scales. A somewhat similar electro-optical device is also disclosed in U.S. Pat. No. 3,453,051 -- Togino in which three photoelectric detectors are used to sense the scale gradations or markings.